1. Field of the Invention
The present invention relates to a fixing member used for a fixing apparatus in an electrophotographic image forming apparatus such as a copier or an LBP, and in particular, relates to a fixing member used preferably in a fixing apparatus of an oilless system. In addition, the present invention relates to a fixing apparatus using such a fixing member.
2. Description of the Related Art
A fixing member is required to have high toner releasability. If it has high toner releasability, when toner images formed electrostatically on a recording material, such as paper, is fixed with heat and pressure, it becomes hard for toner to adhere on a surface of the fixing member. When toner adheres to the member surface, that is, toner offset arises, a problem is raised in that image quality deteriorates due to chips arising partially in a toner image formed on a recording material, or offset toner being re-fixed to the following recording material. Hence, it has been investigated to use a material excellent in toner releasability on the surface of a fixing member, that is, for a releasing layer.
As such a fixing member, one is normally used in which a layer structure composed of one or more layers is formed on the outer periphery of a cylinder shaft, and a fluorocarbon resin is used for a releasing layer.
Although the fluorocarbon resin has low surface energy and is excellent in toner releasability, it has such a demerit that material hardness is high, because it is a resin. If hardness is high, when a toner image formed electrostatically is fixed with heat and pressure, an ability to follow unevenness of paper fibers is low, and hence, it becomes hard to obtain a high quality image. In addition, on a recording material which hardly has convexoconcave, it is difficult to obtain a high quality image because toner particles are crushed more than needed and a small character becomes illegible.
On the other hand, a member using a releasing layer constructed of an elastic material has flexibility in comparison with resins, and has such an advantage that it is easy to obtain a high quality image. Nevertheless, as compared with the fluorocarbon resin, an elastic material, that is, rubber, generally has high surface energy, and is apt to be inferior in toner releasability. In particular, only the fluorocarbon resin is put in practical use as a material used for a releasing layer in color oilless fixing, and a fixing member having a releasing layer constructed of rubber has not been put in practical use up to now. In order to obtain a high quality image, what is desired is not a releasing layer that is constructed of a hard resin, but a releasing layer which is constructed of a flexible rubber.
In addition, since a fixing member is used at a high temperature of about 200° C., a rubber used for a releasing layer is required to have heat resistance. Generally, as types of rubbers which have such heat resistance, fluororubber, silicone rubber, etc. may be cited. Fluororubber refers to a type of rubber, which is an elastic material, produced by adding a cross-linker, a cross-linking assistant, a reinforcing compounding agent and the like to a fluoropolymer which is constructed of partially fluorinated hydrocarbon chains, and forming a three-dimensional network through polyamine cross-linking, polyol cross-linking, or organic peroxide cross-linking.
Heretofore, fluororubber has been used mainly as a releasing layer in an oil coating system in which a denatured silicone oil is externally supplied in a color machine. Types of fluororubber polymers (fluoropolymers) are mainly used which are produced by polyamine-cross-linking or polyol-cross-linking a product produced by blending a metal-containing filler and a reinforcing bulking agent with a fluoropolymer which is constructed of a binary copolymer of vinylidene fluoride and hexafluoropropylene, or a terpolymer of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene.
Silicone rubber refers to a type of rubber, which is an elastic material, produced by forming a three-dimensional network through addition reaction cross-linking, organic peroxide cross-linking or the like of a product produced by blending various fillers as needed with a raw rubber such as polymers which are constructed of methyl-vinyl siloxane units or methyl-phenyl-vinyl siloxane units in which part of methyl groups of methyl-vinyl siloxane units have been replaced by phenyl groups.
Generally, silicone rubber has been primarily used as a thermally conductive elastic material lower layer because of excellent heat resistance. In addition, as a releasing layer, fluororubber has been mainly used in an oil coating system in which dimethylsilicone oil is externally supplied in a color machine.
However, an oilless fixing system with a fluorocarbon resin releasing layer has recently been put in practical use in which, in order to secure releasability, a wax having a releasing assistant effect is internally added to toner, and as mentioned above, no oil coating is externally carried out.
When fluororubber is used for a releasing layer instead of a fluorocarbon resin so as to obtain a high quality image, the affinity between wax in toner and fluororubber was low, and the releasing assistant effect of the wax is not fully exerted. Hence, paper winding arising when toner does not come off a releasing layer, or toner offset has occurred in some cases.
In addition, when dimethyl silicone rubber is used for a releasing layer, the affinity between wax in toner and dimethyl silicone rubber was high. But, the affinity is too high, and a certain adverse effect is exerted on toner releasability, and thus, paper winding or toner offset has occurred in some cases.
It is disclosed in Japanese Patent Application Laid-Open No. 2000-242115 to use a rubber releasing layer in color oilless fixing. The above-mentioned document discloses an image recording device in which a toner containing wax is used, and a releasing layer constructed of fluororubber high in affinity with the wax is formed on the fixing member surface on the side coming in contact with an unfixed toner image on a recording material in a fixing apparatus.
Although, in the above-mentioned document, the releasing layer constructed of fluororubber high in affinity with wax is proposed, this releasing layer is insufficient in toner releasability, and further improvement in toner releasability is desired.
In addition, Japanese Patent Application Laid-Open No. 2004-138956 proposes a fixing belt characterized in that a surface layer is constructed of a vulcanization product of a liquid coating composition having a ternary copolymer of tetrafluoroethylene, vinylidene fluoride and hexafluoropropylene, a vulcanizing agent, and silicone oil. It is described in this document that a silicone oil is preferable which is partly substituted with a fluoro alkyl group, or has at the end of its chemical structure a functional group selected from among a hydroxyl group, an alkoxy group, a carboxyl group and an alkoxy carbonyl group.
Although the above-mentioned document proposes the releasing layer which is constructed of a mixture of general-purpose fluororubber and silicone oil, it has no mention concerning the dispersibility of fluororubber and silicone oil which are low in affinity with each other. If a sea-island structure is taken when the affinity between the two is low, the sizes of islands are uneven. In the case where a particularly large island occurs, a portion with insufficient toner releasability may arise.